The present invention relates to a slack adjuster. A slack adjuster is an integral part of actuating linkage for a vehicle air brake system on heavy duty vehicles, such as buses and trucks. The slack adjuster transmits braking force to a brake shaft, which applies the braking force to a cam and thereby to the brake shoes and drum associated with a road wheel of the vehicle.
One common type of slack adjuster includes a worm and a worm gear that are in meshing engagement within a housing or body. The worm gear is a metal gear that rotates within a cylindrical opening in a metal body. Annular shoulders on the worm gear engage the cylindrical surface of the body, to support the worm gear for rotation within the body. The brake shaft extends through the adjuster and is fixed for rotation with the worm gear by a splined connection. The worm and the worm gear are relatively rotatable to effect adjustment of the linkage to accommodate clearance that develops in the system with extended usage of the brake.
In this type of slack adjuster, the braking force is applied to the body of the slack adjuster through an actuator. The body transmits the braking force, through the worm and the worm gear, to the splined connection with the brake shaft. The brake shaft is thus rotated to actuate the brake.
The worm gear is subjected to the entire braking force passing through the brake adjuster. The resulting force presses the worm gear against the body which causes a substantial amount of friction. This friction in turn causes galling of the body material and worm gear as they interface. When the damage from galling becomes severe the performance of the slack adjuster can be greatly reduced.